ANSI09000023-1-en.vsd
~ ~
Z
A
Z
B
Z
L
A B
IED
I
fA
Fault
ANSI09000023 V1 EN-US
Figure 56: Through fault current from B to A: I
fA
The IED must not trip for any of the two through-fault currents. Hence the minimum
theoretical current setting (Imin) will be:
EQUATION78 V1 EN-US (Equation 21)
A safety margin of 5% for the maximum protection static inaccuracy and a safety
margin of 5% for the maximum possible transient overreach have to be introduced. An
additional 20% is suggested due to the inaccuracy of the instrument transformers under
transient conditions and inaccuracy in the system data.
The minimum primary setting (Is) for the instantaneous phase overcurrent protection is
then:
EQUATION79 V3 EN-US (Equation 22)
The protection function can be used for the specific application only if this setting
value is equal to or less than the maximum fault current that the IED has to clear, I
F
in
Figure 57.
Section 8 1MRK 511 401-UUS A
Current protection
150 Bay control REC670 2.2 ANSI
Application manual
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